If $f:R\to R$ is defined by $f\left(x\right)=\{\begin{array}{ccc}\frac{2\sin x-\sin 2x}{2x\cos x}& \text{if}& x\ne 0\\ \alpha & \text{for}& x=0\end{array}$  then the value of $\alpha$ so that $f$  is continuous at 0 is